Oberth effect Vs Gravity assists

[Red Iron Crown]

Again, dropping from an elliptical orbit and doing an interplanetary transfer burn at an LKO PE only helps harness Oberth if you have low TWR.

The advantage of descending from Minmus or the Mun is that you could have a refueling station there and still have some control over the start of the elliptical orbit.

So if you have a high TWR and aren't planning on refueling, the gain you get from creating an elliptical orbit to "maximize Oberth" will be minimal and is probably not worth the complications of setting it up.

At the risk of throwing out numbers, I just completed a challenge where my design forced me to make an elliptical orbit to maximize Oberth. It used a nuke and took over an eight minute burn for the entire fuel load. Setting up the elliptical orbit netted a gain of around 200 dV for a roughly 3000dV burn (the entire fuel load). So actually not a bad gain. But, if I had 4x the thrust, the burn would have been 2 minutes, I wouldn't have to do the ellipse, and I would have had fuel left over.

If you burned the same dV in a quarter of the time you would have actually gained more due to the Oberth effect due to more of the burn occurring closer to periapsis.

The further away your destination is, the more important maximizing Oberth is. For transfers to Eve or Duna, the gains are minimal. If you're going to Moho or Eeloo, the gains are very significant.

[Claw]

If you burned the same dV in a quarter of the time you would have actually gained more due to the Oberth effect due to more of the burn occurring closer to periapsis.

Yes, this is exactly my point. The concept of Oberth is unrelated to setting up an elliptical orbit first. Setting up an elliptical orbit first is a byproduct of other factors.

(And this is why I dislike throwing around example numbers.)

[Red Iron Crown]

Yes, this is exactly my point. The concept of Oberth is unrelated to setting up an elliptical orbit first. Setting up an elliptical orbit first is a byproduct of other factors.

(And this is why I dislike throwing around example numbers.)

I wasn't clear, sorry. If you had a higher TWR, it would still be worth dropping into an elliptical orbit to harness Oberth, assuming it was worth it at the lower TWR. TWR does not affect the gains from burning at higher speed (other than how closely the burn can be completed to periapsis).

[FREEFALL1984]

Right, so to summarize, taking full advantage of the Oberth effect when in a close high speed orbit is more beneficial than performing a gravity assist around a moon, simply because the fuel I use performing and perfecting my planetary intercept would be more efficiently spent at the higher speeds in orbit around kerbin than in a slower speed orbit around mun or in interplanetary space which would also likely require more corrections than normally necessary.

Thank you gentlemen, I knew the Oberth effect was important but I never realized quite how much of a difference it could make

[adinfinitum]

Right, so to summarize, taking full advantage of the Oberth effect when in a close high speed orbit is more beneficial than performing a gravity assist around a moon, simply because the fuel I use performing and perfecting my planetary intercept would be more efficiently spent at the higher speeds in orbit around kerbin than in a slower speed orbit around mun or in interplanetary space which would also likely require more corrections than normally necessary.

Thank you gentlemen, I knew the Oberth effect was important but I never realized quite how much of a difference it could make

That's a pretty good summary of it. If you wanted to, there is the possibility that you could chain a slingshot from the Mun to Minmus, and that may be more efficient than just burning out of the system. You'd have to either be really lucky or really good at calculating patched conics to get it to work though. Like Red Iron Crown and others have said, if you start from Minmus or Mun orbit and lower your periapsis back to Kerbin, you can save fuel when you've got to do a large burn.

[Geschosskopf]

If I read your earlier post correctly, and you are in orbit around Minmus, then lower your periapsis so that it is near Kerbin to perform your burn at a higher velocity than from an LKO orbit, then it isn't a slingshot.

There is no slingshot unless it's powered. Otherwise it's just a redirection. Gravity alone gives you nothing due to conservation.

Suppose your ship enters an SOI and has a Pe close to the planet. You do nothing, just watch. When the ship 1st enters the SOI, it has a certain velocity. Velocity is a vector composed of the direction the ship is moving in and the scalar speed which is what shows up on the navball. As the ship falls towards Pe, the speed increases and the direction swings around. Aftet the ship passes Pe, however, its speed decreases. Eventually it will exit the SOI with the same SPEED it entered. All the planet did was change its direction. There is no energy gained by the ship. All you've done is change the direction it's going. This is not a slingshot.

It's exactly the same as coasting around an elliptical orbit. That orbit's Ap is directly analogous to the SOI entry and exit points. The ship will always have the same speed at Ap each orbit unless you burn it. So just as a closed orbit isn't a slingshot, so neither is an unpowered open orbit.

A slingshot, OTOH, is when the ship leaves the SOI with more orbital energy than it had when it entered. Because this can't come from gravity, it has to come from the engines. The reason this is called a slingshot is because burning at Pe gives you more energy per ounce of fuel due to Oberth than you would get burning anywhere else.

[Red Iron Crown]

No, a slingshot adds energy because the body has its own motion. No burn is required.

Classic visualization (from Wikipedia): A close terrestrial analogy is provided by a tennis ball bouncing off a moving train. Imagine throwing a ball at 30 km/h toward a train approaching at 50 km/h. The engineer of the train sees the ball approaching at 80 km/h and then departing at 80 km/h after the ball bounces elastically off the front of the train. Because of the train's motion, however, that departure is at 130 km/h relative to the station.

[cantab]

Suppose your ship enters an SOI and has a Pe close to the planet. You do nothing, just watch. When the ship 1st enters the SOI, it has a certain velocity. Velocity is a vector composed of the direction the ship is moving in and the scalar speed which is what shows up on the navball. As the ship falls towards Pe, the speed increases and the direction swings around. Aftet the ship passes Pe, however, its speed decreases. Eventually it will exit the SOI with the same SPEED it entered. All the planet did was change its direction.

This is true in the frame of reference of the planet. It is not true, though, in the frame of reference of the Sun, because the planet itself is orbiting the Sun.

[adinfinitum]

There is no slingshot unless it's powered. Otherwise it's just a redirection. Gravity alone gives you nothing due to conservation.

Momentum is still conserved in a slingshot, the spacecraft gains some and the planet loses some. Because planets are many orders of magnitude larger than a spacecraft, the effect on the planet is negligible. In KSP, because the planets are on rails, there is absolutely no effect on the planets. A gravity slingshot can only be achieved from other bodies orbiting the central body, which is why we can't slingshot around the sun.

Say you enter the SOI of the Mun radially from Kerbin at 400 m/s. Your trajectory then has you exiting the SOI of the Mun on a tangent to its prograde motion. Your velocity is now 400 m/s plus the speed that Mun is traveling. This is equivalent to performing a 400 m/s burn in an orbit that is the same as the Munar orbit, except that you got that 400 m/s for "free".

[Claw]

There is no slingshot unless it's powered. Otherwise it's just a redirection. Gravity alone gives you nothing due to conservation.

The Voyager probe made extensive use. Try this...

http://en.m.wikipedia.org/wiki/Gravity_assist

Conservation does apply, the craft is stealing momentum from the planet. It doesn't really come from gravity, but the body's orbital energy.

[Claw]

Say you enter the SOI of the Mun radially from Kerbin at 400 m/s. Your trajectory then has you exiting the SOI of the Mun on a tangent to its prograde motion. Your velocity is now 400 m/s plus the speed that Mun is traveling. This is equivalent to performing a 400 m/s burn in an orbit that is the same as the Munar orbit, except that you got that 400 m/s for "free".

Your idea is right but I think there's a mistake in the math.The craft didn't gain dV because it turned. It gained a vector adjusted amount of the body's orbital velocity. So in your eample, the "free" gain isn't related to the 400 m/s approach speed.

[Kasuha]

There is no slingshot unless it's powered. Otherwise it's just a redirection. Gravity alone gives you nothing due to conservation.

You know, all you need to check is an actual gravity slingshot. Notice both periapsis and apoapsis of the trajectory after the slingshot are higher, so the resulting orbit definitely has higher energy than the initial one.

Energy is of course conserved (in real world). The slingshot speeds you up (significant effect) and slows down the planet (negligible effect).

Edited March 14, 2014 by Kasuha

[Kryxal]

That's an odd orbit ... did you do an Eve flyby and return or something with multiple orbits?

[WaveFunctionP]

Follow along, and try plugging in numbers of your own if aren't that good at math to get a feel for it. The rest of the series is pretty good at explaining more about the mechanics as well. Edited March 15, 2014 by WaveFunctionP

[NKL]

I just wanna thank you all, for all your explanation.

I was quite sure i had to burn at periaspsis (timing my mid-burn to the node timer, approximately), however I now understand the reason behind such a behaviour.

Also, discovered how to trully profit from slingshot (inclination, a little bit of boost, etc) and I was naivly convinced that we could do both, even if can't try and test myself (not enough patience with stock node system). Thanks to all of you to make my mind about stuff.

I'll humbly had some link, I'm pretty sure everybody here knows, but who knows (credit to the owner of the page):

http://alexmoon.github.io/ksp/

[Pds314]

Basically, because Kerbin is big and the Mun isn't.

The Oberth effect is completely linear. The ideal would be starting moving at very high speed and then instantly accelerating to your new speed. Gravity assists from the Mun can only provide a limited amount of energy. Yes, technically, one can get significant Delta-v from Mun but it is a diminishing return because the Mun has a surface to stop you.

[Mesons]

A few people in this thread have suggested that making the most of the Oberth effect requires 2 burns. There is nothing to be gained by waiting an orbit after changing your orbit's shape--you won't be going any faster when you come back down to periapsis than you were just after your burn. The only exception to this is very long burns, where you need to start burning way ahead of periapsis and burn until way past it. Maximizing your delta-V just requires burning when you're moving the fastest. That's why some people suggest for very long burns to make shorter burns, once per orbit at right around the periapsis.

As for the idea of burning at Mun periapsis, it's better to burn in LKO. The most you can gain from a gravitational slingshot alone is 2* the velocity of the slingshot body, under the right conditions. When you burn close to the Mun, you can gain up to Mun orbit velocity + Mun escape velocity for your burn, but even if you are in a Kerbin orbit at Mun altitude (fastest you can go at Mun altitude without having a higher apoapsis), that speed in the Kerbin SOI (about 1800m/s) is much lower than the ~3100m/s minimum to reach the Mun SOI from LKO.

[FenrirWolf]

Right, so to summarize, taking full advantage of the Oberth effect when in a close high speed orbit is more beneficial than performing a gravity assist around a moon, simply because the fuel I use performing and perfecting my planetary intercept would be more efficiently spent at the higher speeds in orbit around kerbin than in a slower speed orbit around mun or in interplanetary space which would also likely require more corrections than normally necessary.

Thank you gentlemen, I knew the Oberth effect was important but I never realized quite how much of a difference it could make

Bingo. The most important thing to take away from this discussion is that the faster your rocket is going at a given point in time, the more efficient your further fuel useage will be.

[rkman]

So can someone explain why people think the oberth effect is more efficient than a munar gravity assist

I'm not so sure there are many people who think that.

If you want a gravity assist, you still start from LKO, benefiting from Oberth effect and gravity assist.

[Kryxal]

The Oberth effect is easy to take advantage of, just make your burn deep in a gravity well (so you have more kinetic energy) as opposed to higher up ... gravity assists are trickier because they involve SoI and direction changes, but there's no reason you can't do both.

[Claw]

A few people in this thread have suggested that making the most of the Oberth effect requires 2 burns. There is nothing to be gained by waiting an orbit after changing your orbit's shape--you won't be going any faster when you come back down to periapsis than you were just after your burn. The only exception to this is very long burns, where you need to start burning way ahead of periapsis and burn until way past it. Maximizing your delta-V just requires burning when you're moving the fastest. That's why some people suggest for very long burns to make shorter burns, once per orbit at right around the periapsis.

This is exactly what this thread says. Even for shorter burns, you harness more Oberth with multiple burns at the PE. That's not because multiple burns are better for Oberth, but because the reality of TWR means you can't instantaneously apply dV at the theoretical optimum point.